Various multi plate heat exchangers have been devised prior to the present invention to increase their heat exchanger efficiency and usefulness. Among these heat exchangers is the type disclosed in my copending application U.S.S.N. 474,891 filed Feb. 5, 1990 for TUBULAR PLATE PASS FOR HEAT EXCHANGER WITH HIGH VOLUME GAS EXPANSION SIDE, assigned to the assignee of this invention and hereby incorporated by reference. This heat exchanger has a plurality of flattened tubes which are operatively joined at their tank ends to form a core for the passage of volatile heat exchanger fluid therethrough from an intake to an outlet. Each of these tubes has a divider rib to provide a U-flow type passage for the flow of the heat exchanger fluid in each tube with the first or forward course of the U-flow basically conducting the refrigerant in a liquid phase and the back course conducting the refrigerant in the gaseous state. Such heat exchanger tubes are generally rectilinear and are arranged into a core so that the tubes have a thin leading and trailing edges with sides flattened to provide large area heat exchanging surfaces therebetween. This allows the air to flow across the large surface areas with minimized resistance to air flow. With such constructions local dry out areas occur and the heat transfer rate is significantly reduced because the gaseous phase heat transfer efficiency is significantly lower than a liquid or a changing phase mixture of liquid and gas. Accordingly, the front or leading side of the heat exchanger is generally more efficient in heat transfer than the back side thereof with the rib in each tube defining the front and back sides. As a unit, this is caused largely by the fact that the front side of the heat exchanger contains a larger content of liquid than gas as the heat exchanger fluid courses through the core, and accordingly, has a higher potential for heat transfer because of latent heat of vaporization.
In view of the above, this invention provides a new and improved evaporator which features unique U-flow rib or divider construction which eliminates or sharply reduces local dry out areas in an evaporator by improving control of the change in phase from a liquid to a gas as the heat exchanger fluid courses through the heat exchanger from the inlet to the outlet thereof. More particularly, by feeding liquid, or a two phase mixture containing higher quantity of liquid, to the gaseous side of each tube ensures that dry out areas will be significantly reduced and heat transfer efficiency will improve.
Accordingly, it is a feature, object and advantage of this invention to provide a new and improved tube for use in a heat exchanger core in which leak paths are provided from the front side of the tubes comprising the core to the rear side of the tubes so that volatile liquid can short circuit the forward areas of at least some of the tube so as to be available for vaporization in dry out areas, and particularly, in downstream tubes to thereby increase the efficiency of the heat exchanger core.
In a preferred embodiment of the present invention, dry out can be effectively eliminated by providing heat exchanger fluid bypass channels in the divider rib. These channels are sized to eliminate back flow, as well as to keep the lower part of the plate fed with refrigerant since a large bypass channel may short circuit too much of the liquid refrigerant. Also, towards the lower part of the tube, these channels may be progressively smaller in size and flow capacity as the need for bypassing liquid refrigerant reduces. For ease of manufacture, these channels may be substantially the same size and still provide improved efficiency. These bypass channels may be important in the evaporator with an expanding freon path as in my prior application, identified above, since the likelihood of local dry out is substantially higher.